Most prior attempts to create color images of photographic quality using the science of electrophotography have employed liquid developers. For many years it was thought that liquid developers were the only developers with fine enough particles to give the resolution ordinarily experienced in silver halide photography. Recently, multicolor images have been formed using toner particles finer than 8 microns in diameter and in some instances finer than 3.5 microns in diameter (mean particle size). With such size particles, granularity comparable to silver halide photography is obtainable.
Unfortunately, transferring a high percentage of an image of fine toner particles with good resolution is much more difficult than with larger toner particles. We have found that traditional electrostatic transfer is not particularly effective at such particle size.
U.S. patent application Ser. No. 230,394 to Rimai et al, entitled THERMALLY ASSISTED TRANSFER OF SMALL ELECTROSTATOGRAPHIC TONER PARTICLES, filed Aug. 9, 1988, now U.S. Pat. No. 4,927,727, issued May 27, 1990, describes a method of transferring toner particles by heating a receiver to a temperature which sinters the toner particles causing them to stick to each other and to the receiver thereby effecting transfer of the toner from the element to the receiver. According to a preferred embodiment, the receiver is coated with a thermoplastic polymer with a glass transition temperature such that it becomes soft in the process and the toner particles are at least partially embedded in it to assist in the transfer. According to that embodiment, the first layer of toner being transferred partially embeds itself in the thermoplastic polymer on the receiving sheet while the second and subsequent layers stick to each other and to the first layer. Thus, the thermoplastic polymer becomes soft enough to accept some embedding and the toner is heated sufficiently that discrete toner particles stick together at some points of contact. The preferred method of heating the receiver is to preheat the thermoplastic surface prior to the nip although heating a backing roller is also suggested as a less advantageous alternative or in conjunction with external heating.
U.S. patent application Ser. No. 07/405,258 to Rimai et al, filed Sept. 11, 1989 suggests that to prevent curl of such thermoplastic coated receiving sheets after finishing, a thermoplastic layer should also be coated on the side of the receiving sheet opposite that receiving the image. To aide handling of the sheet the thermoplastic chosen for this opposite side is polyethylene or polypropylene which are relatively high melting point thermoplastics which are less likely to soften and not release from surfaces supporting the back of the receiving sheet.
Multicolor toner images are generally formed by the transfer of 3 or 4 single color images in registration to a receiving sheet. High-quality resolution is presently only possible with such a multiple transfer approach by mounting the receiving sheet on a roller and rotating the roller to bring the receiving sheet through transfer relation with the consecutive single color images to transfer them in registration. A radiant preheating device above and out of contact of the receiving sheet as it enters the nip while carried by the roller is an effective and desirable approach to practicing the invention of the above application. However, temperature control and heat utilization efficiency is much easier if the roller itself provides the heat. Heating the roller effects substantial preheating as suggested in the prior application to the extent that the entire roller is heated not just the portion in the nip.
To attempt to practice thermally-assisted transfer of small toner particles with the primary heat coming from within the transfer roller, certain problems occur. An aluminum core roller with a thin layer of hard silicone rubber was heated by a conventional fusing lamp inserted inside the roller. Satisfactory transfer could be obtained of 3 and 4 color images providing very low glass transistion temperature toners were used. However, with higher glass transition temperature toners, both blistering of the paper and offset of the backside coating onto the roller occurred.